It’s not a global variable; in fact it’s not a variable at all. It’s just placeholder within the sprintf() function. Take a look at the sprintf PHP function documentation.
In the example you cite, the author is using the ‘argument swapping’ placeholder syntax: %n$t where %n is the placeholder number that corresponds to the argument order (eg: %1, %2 etc) and $t is the type indicator, where ‘t’ is one of ‘s’, ‘d’, ‘f’ etc (eg: $s, $d etc). Put them together and you get %1$s %2$d etc.